cmd/compile: implement comparisons directly with memory

Allow the compiler to generate code like CMPQ 16(AX), $7

It's tricky because it's difficult to spill such a comparison during
flagalloc, because the same memory state might not be available at
the restore locations.

Solve this problem by decomposing the compare+load back into its parts
if it needs to be spilled.

The big win is that the write barrier test goes from:

MOVL	runtime.writeBarrier(SB), CX
TESTL	CX, CX
JNE	60

to

CMPL	runtime.writeBarrier(SB), $0
JNE	59

It's one instruction and one byte smaller.

Fixes #19485
Fixes #15245
Update #22460

Binaries are about 0.15% smaller.

Change-Id: I4fd8d1111b6b9924d52f9a0901ca1b2e5cce0836
Reviewed-on: https://go-review.googlesource.com/86035
Reviewed-by: Cherry Zhang <cherryyz@google.com>
Reviewed-by: Ilya Tocar <ilya.tocar@intel.com>

1 files changed, 1 insertions, 1 deletions

diff --git a/src/cmd/compile/internal/ssa/schedule.go b/src/cmd/compile/internal/ssa/schedule.go
index c44c243eac..8109908475 100644
--- a/src/cmd/compile/internal/ssa/schedule.go
+++ b/src/cmd/compile/internal/ssa/schedule.go
@@ -264,7 +264,7 @@ func schedule(f *Func) {
 			}
 		}
 		if len(order) != len(b.Values) {
-			f.Fatalf("schedule does not include all values")
+			f.Fatalf("schedule does not include all values in block %s", b)
 		}
 		for i := 0; i < len(b.Values); i++ {
 			b.Values[i] = order[len(b.Values)-1-i]